(1) Field of the Invention
The invention relates to a method of preventing poisoned via metallurgy, and more particularly, to a method of preventing poisoned via metallurgy by adding a nitrogen plasma treatment for a spin-on-glass layer in the fabrication of integrated circuits.
(2) Description of the Prior Art
The spin-on-glass materials have been used for planarization of integrated circuits. The material to be applied is thoroughly mixed in a suitable solvent. The spin-on-glass material suspended in the vehicle or solvent is deposited onto the semiconductor wafer surface and uniformly spread thereover by the action of spinning the wafer. The material fills the indentations in the integrated circuit wafer surface; that is planarization. Most of the vehicle or solvent is driven off by a low temperature baking step. Other coatings of the spin-on-glass material are applied and baked until the desired spin-on-glass layer is formed.
The final step in the making of the spin-on-glass layer is curing. Curing is a high temperature heating step to cause the breakdown of the silicate or siloxane material to a silicon dioxide like cross linked material. A cured spin-on-glass process is described in U.S. Pat. No. 5,003,062 to Yen.
In the conventional etchback process as taught by Yen U.S. Pat. No. 5,003,062, the spin-on-glass layer is etched back resulting in a poisoned via metallurgy, such as aluminum, caused by outgassing. Referring now to FIG. 1, there is illustrated a partially completed integrated circuit in which a first metal layer 12 has been deposited on a semiconductor substrate 10. The intermetal dielectric layer is typically composed of a spin-on-glass layer 16 sandwiched between silicon oxide layers 14 and 18. When a via opening is made through the intermetal dielectric sandwich layer, moisture 20 is absorbed from the atmosphere by the exposed spin-on-glass on the sidewalls of the via opening. After the second metal, typically aluminum 22, is deposited as shown in FIG. 2, the moisture 20 is released from the spin-on-glass layer 16. This moisture diffuses to the interface between the first and second metal layers causing poisoned via metallurgy.
Various methods have been suggested to overcome the poisoned via problem. U.S. Pat. No. 5,219,792 to Kim et al teaches covering the spin-on-glass edge within the via with a silicon oxide layer to prevent outgassing. "Hot-Carrier Aging of the MOS Transistor in the Presence of Spin-On Glass as the Interlevel Dielectric" by Lifshitz et al, IEEE Electron Device Letters, Vol. 12, No. 3, March 1991, pp.140-142, suggests the use of a silicon nitride cap on the cured spin-on-glass to overcome aging of the MOS transistor. "An Advanced Interlayer Dielectric System with Partially Converted Organic SOG By Using Plasma Treatment," by M. Matsuura et al, Jun. 8-9, 1993 VMIC Conference, teaches curing a spin-on-glass layer in an oxygen plasma followed by curing in a nitrogen plasma to form an inorganic spin-on-glass layer on an organic spin-on-glass layer.